Interpretive Summary: Epidemiological studies show that foodborne illnesses caused by Escherichia coli O157:H7 have been associated with contaminated meat grinders. However, there is inadequate information about the effects of meat grinders on the distribution of E. coli O157:H7 in ground beef products. This study was designed to characterize the effects of a mid-scale grinder on the distribution of E. coli O157:H7 in ground beef when a single contaminated beef trim was introduced into a stream of uncontaminated beef trims. Six levels of E. coli O157:H7 were evaluated in different batches of ground beef for the presence and quantity of E. coli O157:H7. It was found that E. coli O157:H7 on a small trim is distributed by the grinder over a large quantity of meat. An exponential relationship was determined between the initial contamination on the beef trim and the volume of contaminated ground beef. Distribution models based on a Chi-squared algorithm were developed for each inoculum level describing the contamination level as related to the batch fraction processed. The remnant contamination detected in the grinder components corroborates that when beef processors are testing for pathogenic contamination in a mid-size grinder they should test the beef residues in the collar. Also, sanitation operations of the grinder should include a complete removal and thorough cleaning of the collar. The models developed could be used to estimate the size of the contaminated batch and the pathogen distribution in the batch.

Technical Abstract:
The purpose of this research was to develop models that describe the amount and distribution of ground beef contaminated with Escherichia coli O157:H7 when a contaminated beef trim is introduced into a batch of uncontaminated beef prior to processing in a mid-size commercial grinder (34 g/s). A beef trim (0.6 plus/minus 0.1% of the batch) was inoculated with a rifampacin-resistant strain of E. coli O157:H7 and added to a batch of non-contaminated trims at the grinding step. Six inoculum levels (1, 2, 3, 4, 5, and 6 log CFU) were tested in this part of the study. Extensive grinding treatments were conducted with inoculum levels of 2, 3, and 4 log CFU using an additional mass of 37% more beef trims to the batch to determine the pathogen removal in the grinder by the ground beef. All the ground beef was tested for E. coli O157:H7rif. E. coli O157:H7rif was detected in 9 to 86% of the total ground beef for the 1 to 6 log CFU inoculum levels, respectively. Even though, at the lower inoculum levels (2 and 3 log CFU) tested with the extensive grinding, E. coli O157:H7rif was not detected in the hub, screw and beef leftover in the grinder, a remnant contamination was detected in the collar that fixes the grinder's die and blade to the hub. An exponential algorithm described the relationship between the quantities of ground beef containing E. coli O157:H7rif and the inoculum level (R2=0.824). Distribution models based on a Chi-squared algorithm were developed for each inoculum level describing the contamination level as a function of the volume of the batch processed (R2 from 0.814 to 0.999). The results of this study corroborate that when beef processors are testing for pathogenic contamination in a mid scale grinder they should test the beef residues in the collar. Also, sanitation operations of the grinder should include a complete removal and thorough cleaning of the collar. The models developed could be used to estimate the size of the contaminated batch and the pathogen distribution in the batch.